One thing you can’t really get outside of Korea is the BBQ. It is characterized by food cooked at the table and served with unlimited side-dishes. In Korea, authenticity is very important, and nobody would claim their BBQ was authentic if they cooked the food in the kitchen and brought it out to the table, as is common practice at Korean restaurants in the US.
In my hometown, there is one Korean restaurant that cooks at the table. Most reviews give it more $-signs than starts, and the general sentiment is that if you’ve ever been to Korea, you will be disappointed even if it were free. Meanwhile, there are restaurants on every corner in Seoul that serve BBQ for less than the cost of a mediocre hamburger in any US metropolis.
In conclusion, it is important to eat as much BBQ as possible in Korea.